Cells in a rechargeable battery pack have variations in charging capacity, discharging capacity and/or state of charge. For optimal use, the cells should be balanced. To balance cells, the voltage across each cell or parallel group of cells is measured. In passive balancing, cells or groups of cells with higher voltage are then discharged until their voltages match that of the other cells. Cell discharge current is usually routed through a discharge resistor. Electrically noisy operating environments, such as in electric and hybrid vehicles, make measuring the voltage across each cell or group of cells difficult. This can cause errors in determination of whether or not a cell should be discharged or how much the cell should be discharged, when balancing. Frequent incomplete charge and discharge cycles, such as encountered with electric and hybrid vehicles, can exacerbate cell differences in state of charge. Also, an electrical fault such as a disconnected or burned out discharge resistor or a broken switch could be present in a battery balancing circuit, and this situation could go undetected. Some types of batteries, if not properly balanced, can fail catastrophically.
It is within this context that the embodiments arise.